An accurate understanding of diode properties can help improve circuitry that involves diodes. Appropriate circuitry can improve the functionality of coils, make circuits more robust to high voltages, and reduce construction time. An unbiased reverse diode pair (RDP) can tolerate high voltage RF waves but will introduce distortion and high resistance to RF waves; and, a forward-biased (FB) diode has negligible distortion of RF waves and can handle very high voltage RF waves without diode breakdown, but is very susceptible to breakdown when unbiased. Both methods have been successfully used separately in transmit and receive coil decoupling circuits, as well as with many other circuits, including transmit-receive (TR) switches and multi-tuned coils.
An important feature of PIN diodes is their ability to remain forward-biased with large RF signals and small DC bias current. The maximum RF current that the diode can control depends on the amount of stored charge supplied by the DC forward-bias relative to the charge variations produced by the RF signal. When a diode is forward-biased with current IF, the I-region has a stored charge of Q=IDCτ, where τ is the recombination time or carrier lifetime. The diode's resistance is inversely proportional to Q. The charge, q, introduced by RF during a half cycle is q=IRF/πf. In order for the diode to remain on, Q must remain greater than q, or, in another form, IDCτ>IRF/πf. This theory predicts that a diode with 100 mA of DC current, a carrier lifetime of 1.0 μs, can handle a 100 MHz wave with current up to 30 A. PIN diode RF capabilities can be greater than 2000V and 25 A. Leenov (The silicon PIN diode as a microwave radar protector at megawatt levels. (Electron Devices, IEEE Transactions on 1964; 11(2):53-61)) tested PIN diodes biased with 100 mA of current, using 2.5 μs pulse length at 9.0 GHz, with 70 kW of power, and predicted and experimentally verified that a PIN diode can handle 38 megawatts of power for 1 μs, and tens of kilowatts continuously.
The present invention is a methodology utilizing a forward-biased reverse diode pair (FB-RDP) which has negligible RF distortion, very high RF voltage tolerance, low resistance to RF, and is not as susceptible to breakdown when unbiased. A FB-RDP can be implemented in most instances where there is a single diode, improving the robustness of the circuit.
The present invention represents a departure from the prior art in that the methodology of the present invention allows for a more robust construction of electronic circuits using readily available components while simultaneously reducing RF distortion.